A push button dialing type general telephone has a DTMF signal generating means. A DTMF signal is formed by combining two frequencies, each of which is selected from a high frequency group of 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz and a low frequency group of 679 Hz, 770 Hz, 852 Hz and 941 Hz, respectively. Accordingly, a combination of 4 by 4 equaling to 16 different DTMF signals is possible for the signal generation. In general, these 16 different DTMF signals are allotted to respective numerals of “0” through “9” and codes of “*”, “#”, “A”, “B”, “C” and “D”. The user is allowed to receive such services as telephone exchange connections and the like by sending dial signals via a general telephone to a telephone exchange of the public telephone network or an analog type private branch exchange by means of the DTMF signals.
Further, a general telephone is provided with function keys such as a operation keys, a redial key and the like in addition to dial keys for outputting the DTMF signals, thereby allowing such telephone functions as one-touch dialing, abbreviated dialing, redialing and the like to be performed.
Recently, many private branch exchanges have been employing a digital system and the ones with more advanced functions have been prevailing. However, since the aforementioned general telephone is designed for connecting with general lines (such as an analog station line, a digital station line and the like), the general telephone as such some times does not work properly or can not utilize well the service functions of a private branch exchange.
In addition, a private branch exchange is designed on the understanding that numerous service functions are realized by having a dedicated telephone of the private branch exchange connected, thereby allowing many of the private branch exchanges to be satisfied with performing a minimum of functions when a general telephone is connected therewith even if the connection is possible.
Therefore, when a general telephone is connected to a private branch exchange as one of the extension stations together with dedicated telephones, the user of the general telephone is restricted in utilizing the numerous service functions that are made available to the user of a dedicated telephone, thereby preventing the user from enjoying the valuable and diversified service functions of the private branch exchange or forcing the user to go through complex steps of maneuver even if the general telephone can be connected. Even when operational instructions are exchanged between a general telephone and a private branch exchange by using the foregoing DTMF signal system, despite the framework of 16 different signals that are possible theoretically, it is usual that the numerals of “0” through “9”, and codes of “*” and “#” only are used and the codes of “A”, “B”, “C” and “D” remain unused.
Furthermore, many of the push-button type general telephones have no buttons for sending out the codes of “A”, “B”, “C” and “D”. One of the reasons for this is considered to be lack of a sufficiently improved infrastructure (including private branch exchanges), whereby services are provided through the use of DTMF signals of “A”, “B”, “C” and “D”.
Accordingly, the combined use of prior art push-button type general telephones and a private branch exchange has not allowed the functions of the private branch exchange to be exploited to the fullest extent. As a result, when an enterprise employing no private branch exchange as yet decides on an introduction of a private branch exchange, for instance, it is absolutely needed that dedicated telephones are procured at the same time to exploit the functions of the private branch exchange sufficiently. And, the same can be said when an enterprise having already installed a private branch exchange decides on replacing the private branch exchange. In such cases, the old type telephones that have been long used are often discarded.
On the other hand, with respect to a general telephone itself, the DTMF signals therefrom are generated generally by an LSI provided with a DTMF signal generating function and many of the general telephones are missing the buttons to send out the codes of “A”, “B”, “C” and “D” in spite of the fact that the LSI is capable of sending out the numerals of “0” through “9” plus codes of “*”, “#”, “A”, “B”, “C” and “D”, thus having the inherent functional capabilities left unused.